Field of the Invention
The invention relates to a lithium ion battery membrane, and more particularly to a high performance coated membrane for power batteries. The coated membrane features excellent compressible elasticity, thermal shutdown, low thermal shrinkage, and high temperature resistance, which are favorable to the improvement of the safety and cycle performance of lithium ion batteries.
Description of the Related Art
Polyolefin microporous membrane has nano micropores (having an average pore size of less than 300 nm, a porosity of 30-65%, and a normal thickness of 16, 20, and 25 μm) in the form of a penetrable three dimensional network and features high voltage oxidation resistance and stability in organic electrolytes of lithium ion batteries. As a membrane material, the polyolefin microporous membrane has been widely applied to batteries of cell phones and laptops. Typical commercial polyolefin microporous membranes include: a three-layer of PP/PE/PP composite membrane prepared by a dry method, and a single layer of PE membrane having a large molecule weight prepared by a wet method. Including the polyolefin microporous membrane, the two membranes feature proper thermal shutdown temperature, which can satisfy the requirement of the lithium ion batteries for the electronic products. However, the membrane for the power battery is much strictly required. For pore size control, the membrane should have reasonable nanoscale pore size, highly uniform pore size distribution, relatively high porosity, and uniform distribution; in the mechanical aspect, the membrane should have relatively high toughness and tensile strength in the transverse direction and excellent compressible elasticity in the thickness direction, and high compression resistance and local acupuncture strength; in the aspect of the electrical property, the membrane is required to have high insulation breakdown voltage and low ion impedance; and in the thermal performance, the membrane is required to have stable size at high temperatures, low thermal shrinkage, shutdown property, and high temperature rupture resistance. Besides, the membrane is also required to have excellent electrolyte wettability, and improved imbibition and solution retention capacities.
The current commercialized uniaxially stretched membrane prepared by the dry method presents the following shortages when applied to the power battery:
1. The membrane has deficient toughness, and a transverse elongation at break thereof is less than 25%, thereby being easily torn in the transverse direction.
2. Although the middle microporous layer employs PE that has the shutdown temperature of 135-145° C., large thermal shrinkage and membrane rupture at high temperature resistance still exist in the membrane at the temperature of above 120° C.
3. Compressible elasticity and stress absorbency are deficient in the thickness direction.
To improve the transverse rupture resistance of the PP/PE/PP membrane prepared by the dry method, Chinese patent publication number CN 02152444.0 discloses a method including blending less than 10 wt. % of thermoplastic polyolefin elastomers, i. e., ethylene propylene monomer (EPM) and ethylene propylene diene monomer (EPDM), into a polyolefin matrix, and stretching a resulting mixture to produce pores. However, the nature of the thermoplastic polyolefin elastomers determines the formation and distribution of crazes in the polyolefin matrix during the cold stretching, that is, the capability of pore-formation through stretching polyolefin matrix using the dry method is influenced, and the proper porosity cannot be obtained. Thus, the proportion of the added thermoplastic polyolefin elastomers must be as small as possible. As a result, the elastic property is improved slightly, resulting in low applicability.
To improve the uniaxially stretched membrane prepared by the dry method, Asahi Kasei, Tonen Chemical, and other companies have adopted thermally induced phase separation to develop the biaxially stretched nano microporous membrane having high molecular PE prepared by the wet method since 1990s. The raw materials for the membrane generally have a weight average molecular weight of above 500000, and the membrane prepared by the wet method has obviously improved transverse tensile strength and elongation at break compared with the membrane prepared by the dry method. However, the current PE membrane prepared by the wet method applied to the power battery has the following shortages:
1. The membrane has relatively large thermal shrinkage at above 120° C. and deficient high temperature rupture resistance.
2. The membrane also lacks absorbency in the thickness direction and cannot satisfy the high requirements on the security and the cycle life of the power battery.
Chinese patent application numbers 200680035668.3, 200780005795.3, and 200510029794.5 also disclose the co-extruded membrane preparation in the polyolefin composite membrane prepared by the wet method, which primarily include regulating the solid content of the polyolefin raw material and the polyethylene/polypropylene ratio and controlling the molecular weights of raw materials for different layers to obtain different interlayer porosities, pore size distributions, and melt points of different membrane layers. However, such co-extruded composite membranes have defects in improving the membrane rupture at high temperature, the compression resistance, and the resilience of the membrane.
In addition to the polyolefin microporous membrane, there is a microporous physical gel membrane prepared by the solvent-induced phase separation method, such as the PVDF-HFP copolymer microporous physical gel membrane prepared by Bellcore process. The microporous physical gel membrane when in use is cohered to pole pieces by hot pressing process to form integral pole pieces. The PVDF-HFP porous membrane after imbibition ensures uniform distribution of the electrolyte between the pole pieces. The gel membrane is also superior to the non-polyolefin membrane in compressible elasticity, imbibition, and solution retention capacity, and the cycle life of the prepared polymer battery is relatively long at low temperature and the room temperature. However, the copolymer is partially dissolved at 60° C. and the cycle life of the battery is poor at the high temperature. Besides, the PVDF-HFP copolymer gel membrane has slightly larger average pore size, approximately 0.3-2 μm, and low mechanical strength. The microporous gel membrane cannot adopt the hot stretching strengthening process that is commonly used for the polyolefin microporous membrane and is inadaptable to highly efficient battery rolling process. Even lamination process is adopted, it is also required to improve the thickness (generally designed to be 50-70 μm) of the membrane to prevent the short circuit of the battery. The larger the thickness of the membrane is, the larger the resistance of the electrolyte between the positive pole piece and the negative pole piece is, which is not beneficial for the rate capability and the energy density of the battery. Due to the high production cost and the limit energy density of the membrane, the membrane has not been widely applied to the lithium ion batteries but only a small amount of them are applied to special technical fields.
To improve the high temperature shrinkage resistance and the high temperature rupture resistance of the current polyolefin microporous membrane, Chinese patent application numbers 200880003493.7, 200880000072.9, and 201010022936.6, and LG Chem disclose technical solutions including coating microporous ceramic coating layer on the surface of the polyolefin microporous membrane to form a composite membrane. The composite membrane is capable of bearing penetration of the thermocauter at 200° C. Some foreign lithium ion companies adopt the method of coating the ceramic layer on the surface of the membrane to improve the high temperature shrinkage property and the high temperature rupture resistance, and such method are specifically introduced in U.S. Pat. No. 7,892,673 B2, U.S. Pat. No. 6,447,958 B1, U.S. Pat. No. 7,883,799 B2, and U.S. Pat. No. 6,432,586. AI2O3 is the most common inorganic ceramic material, and the membrane surface after coating such inorganic ceramic material has improved thermal shrinkage and rupture temperature. However, compressible elasticity and solution retention capacity of the ceramic coated membrane are not good, and the cycle performance of the battery is not obvious improved. In addition, the ceramic coating lacks flexibility and cannot be too thick, and the coating layer is easily separated from or peeled off the membrane in practical production and application.
Asahi Kasei also announced that they successfully developed the inorganic hybrid membrane IBS with high output power application in 2008. The porosity of the membrane is increased by 50-70%, the resistance thereof is decreased to half of that of the traditional product, and the acupuncture strength thereof reaches above 4.9 N(500 gf).
Battery membrane “Separion” based on the organic substrate of non-woven fabric combined with an inorganic coating layer manufactured by Degussa Company from Germany possesses both the flexibility of the organic compound and the thermal stability of the inorganic ceramic, and the high temperature resistance thereof reaches 200° C. In the charging-discharging process of the battery, even the organic substrate melts, the inorganic coating layer remains the integrity of the membrane so as to prevent large area short circuit, and such an organic/inorganic composite membrane provides an operable solution for the high temperature resistance of the membrane. But the non-woven fabric has large pore size, and the short circuit and self-discharging phenomenon are serious, and besides, such organic/inorganic composite membrane does not have the high strength and proper thermal shutdown temperature specially possessed by the polyolefin microporous membrane, thereby resulting in poor security of the battery.
Chinese patent application number 200510086061.5 discloses a technical solution for preparing a microporous coating layer on the surface of the polyolefin microporous membrane using high temperature resistant polyamide, polyamideeimide, polyimide having melt points of exceeding 180° C. Chinese patent application number 200480034190.3 proposes a technical solution including coating a gelled fluororesin on the surface of the polyolefin microporous membrane to form the coating layer. The technical solutions also have the same shortages as the ceramic coated membrane, which are specifically as follows: 1. Because the current polyolefin membrane basically belongs to the inert material, the bonding force of the polyolefin membrane to the coating layer is not enough, too thick coating results in easy separation from the membrane, and too thin the coating inhibits the thermal shrinkage function of the polyolefin membrane. 2. Capillary action exists in the micropores of the polyolefin membrane. The gel in the slurry easily enters the micropores of the polyolefin membrane when coating the composite membrane, which may affect the pore size distribution of air permeability of the membrane after solvent evaporation, desiccation, and formation of the membrane. The consistency of the membranes produced in batches using the coating method is difficult to control, and the production costs thereof are high.
All the above are about formation of a high temperature resistant coating layer on a polyolefin microporous membrane using the coating method. In contrast to the technology of coating the ceramic layer on the membrane surface, Matsushita from Japan and other companies have proposed a technical route of coating the ceramic layer on the surface of the negative pole piece of the battery so as to avoid the shortages existing in preparation of the high temperature resistant microporous ceramic coating layer on the polyolefin membrane surface. The Matsushita company has tried to coat the AI2O3 microporous coating layer having a thickness of 1-2 μm on the surface of the negative pole piece surface of the electrical core 18650 having a capacity of 2600 mAh and put into production and application in batches since 2007 after the Sony laptop battery accident. However, the pole piece is relatively thick, the load for drying the coating is large, the length of each reel in practical production is limited, and the production cost is relatively high. Besides, the ceramic coating layer does not possess the compressible elasticity and the improvement in the cycle life of the battery is not obvious.
Yang Li, Wang baofeng, et al., from Shanghai Jiaotong University have proved from experiments that periodic changes occurs in the volume and pressure of the graphite anode materials during the charge and discharge processes. Chines patent application numbers 200680010010.7, 200680010890.8, 200680010912.0, and 200680031471.2 disclose technical solutions for regulating the hot stretching. However, the compressible elasticity of the membrane is insufficiently improved, and the membrane has a certain thickness change rate only in conditions of high compressive stress and high temperature 2.2 megapascal/90° C., which cannot satisfy the practical application requirements of the battery. Generally, the compressive stress between the pole piece and the membrane does not exceed 0.7 megapascal. Except the high temperature of 85-90° C. is used for dehydration before the injection of the battery, a normal service temperature of the battery is between −20 and +60° C. Thus, the membrane is required to adapt to the compressible elasticity in normal charging-discharging conditions within a normal service temperature range.
To overcome limitations of the product and the technology of the existing membrane, a technical solution is proposed in the invention that a coated membrane is prepared by coating composite material of ceramic powder and the rubber particles on one side of the polyolefin PE microporous membrane. The ceramic powder and the pre-crosslinked rubber particles are combined together by an aqueous adhesive to form the coating layer on the PE membrane. The microporous membrane based on the high-density polyethylene (HDPE) having different pore sizes on two sides and being biaxially stretched and strengthened is used as the substrate of the coated membrane. The coating layer is coated on the side of the substrate having larger pore size, thereby being beneficial for improving the adhesive strength between the coating layer and the substrate. The PE substrate of the coated membrane has a proper thermal shutdown temperature of 125-145° C., and a stretched and strengthened PE substrate has high mechanical performance. As the coating layer material has proper compressible elasticity, thermal shrinkage resistance at high temperature, high temperature rupture resistance, and fast imbibition, the coated membrane with complementary functions is adapted to requirements of high security, high cycle life of the lithium ion battery. The rubber particles having pre-crosslinked submicron particle size are utilized as the coated membrane as the raw material. The rubber particle mainly enhances the imbibition and the compressible elasticity, and further inhibits the high temperature shrinkage and high temperature rupture phenomenon of the PE substrate together with the ceramic powder. The rubber latex is used as the raw material and treated with irradiation crosslinking so as to obtain the pre-crosslinked rubber particles having controllable particle sizes in the aqueous solution, thereby preventing the second agglomeration and particle enlargement existing in the rubber powder prepared by atomized drying and meanwhile making the coating layer have uniform nanoscale thickness. Post-crosslinked rubber has imbibition property in the proper organic solvent and is superior to the non-crosslinked rubber in compressible elasticity. The ceramic powder is adopted by the coating layer as a separator for separating the rubber particles, thereby preventing the rubber particles from forming impermeable rubber layer. The ceramic powder existing in the coating layer also inhibits the high temperature thermal shrinkage and high temperature rupture of the PE substrate. The rubber particles and the ceramic powder adopt the aqueous adhesive to form the composite coated membrane based on the PE microporous membrane, such a coated membrane has complementary functions, simple process, uniform thickness, and low production cost. The coated membrane prepared by the technical route of the invention possesses the above characteristics and enables the power battery to have significantly improved security and cycle performance. The coated membrane also has controllable production cost in large-scale production and strong market competitiveness.